A regulated firm that arranges cover for you, usually paid commission by the insurer.
A broker sits between you and the insurers, gathers your requirements and arranges the policy. Brokers are authorised and regulated by the FCA and must tell you how they are paid. Most are paid commission by the insurer, which is included in the premium rather than charged to you separately. The Health Guide is published by a broker, PremierPMI, and explains that relationship on its funding page.